We are investigating the adrenocortical response to psychological stress in one year old infants. The infant is stressed by separating him from his mother for an hour. Cortisol excretion levels in response to separation anxiety are compared with normal levels. Urinary free corticoids are determined by the competitive protein-binding radioassay method of Murphy. The objectives of the research are: 1) to determine whether infants at this age react to psychologically meaningful stress with altered endocrine responses in ways similar to adults; 2) to examine if individual differences in response to stress and defense against anxiety are reflected in the hormone levels; and 3) to estblish a methodology for studying other such psychobiological relationships.